1. Field of the Invention
This invention relates to a method and apparatus for controlling the gain of an RF amplifier in response to noise, missing pulses or signal loss and providing a real-time indication of the RF telemetry signal strength to the user of a medical device programmer.
2. Description of the Prior Art
In the field of programmable implanted medical devices, such as cardiac pacemakers, tachyarrhythmia-control devices, implantable drug dispensers and nerve stimulators, it has become common to provide an interactive, transceiver system for both remotely programming operating functions, modes and parameters of the implanted device, and telemetering out data related thereto on command by RF telemetry. In nearly all such active, electronic, implanted medical devices, it has become highly desirable to have the ability to reprogram the device's modes of operation, parameters and other functions and to monitor the performance of the device, both historically and contemporaneously. Such current medical devices are designed to provide two-way telemetry by radio frequency signal transmission between the implanted device and the programming head or wand of the external programmer to provide for the exchange of binary coded transmitted information to enable the aforementioned programming by telemetry-in and the reading out of data stored in the device by telemetry-out.
For example, the Medtronic U.S. Pat. No. 4,253,466 describes an implantable digital, programmable, cardiac pacemaker pulse generator which may be programmed by the programmer described in Medtronic U.S. Pat. No. 4,250,884, both incorporated herein by reference. Such programmers as disclosed in the '884 patent are microprocessor-based and menu-driven under an overall operating routine and subroutines. Such programmers display operating conditions, commands and error messages to prompt the user in the proper use of the programming system. The system of the '466 and 884 patent sets forth the software-controlled programming of a series of Medtronic.TM. pacemakers, but it does not describe the telemetry-out on command of the external programmer of the programmed commands, electrogram, end of-life indicators and the like that have been developed and implemented in subsequent programming systems.
More recently, microprocessor based programmers have been developed by Medtronic, Inc. and others which are operated under the control of dedicated, plug in ROM modules to enable the operation of the system with regard to specific model or series of models of implanted pulse generators. In such systems, the programmer is incapable of operating until a plug-in module or cartridge has been properly installed. The Medtronic.RTM. MemoryMod.TM. cartridge enables the physician to apply the programmer to a specific set of pulse generator models. The software cartridge concept allows Medtronic, Inc. to expand and update the application of the programmer to new pulse generators and functional capabilities as they become available.
Referring now to FIG. 1-4, the Medtronic Model 9710, 9710A or 9710E programming system (hereinafter the Model 9710 system) is depicted. In FIG. 1, the programmer 10, printer 12, programming head 14 and ECG cable and electrode leads assembly 16 is depicted in a simplified form. The MemoryMod cartridge is installed in programmer 10 which includes within its case the micro processor based computing system, an LCD display, a keyboard, and an AC or battery power supply. The programmer 10 is connected to a printer 12 in order to provide a printed record of values programmed into the implanted device or received from the implanted device or ECG cable and electrode leads assembly. Compatible Medtronic printers include the Model 9712 printer and the Model 9751 system enhancement module high-speed printer. The programming head 14 must be connected for all programming and telemetry functions to the programmer 10 and will be described in greater detail hereafter. The ECG cable and electrode leads assembly 16 is required for the functions that detect the electrocardiogram and/or pacemaker signals via skin electrodes, including the "automatic threshold," "measure," and programming confirmation by way of the program confirmation indicator emitted by the pacemaker (as an alternative to confirmation by telemetry). The processing of input commands applied by way of the keyboard on the programmer to the programming system, the formatting of data for telemetry via the programming head 14, and the processing of signals picked up from skin electrodes via ECG cable and electrode leads 16 is under the control of the software contained within the MemoryMod cartridge. The programming head 14 may comprise the Medtronic Model 9713 or 9713D programming head.
In FIG. 2, the ECG cable and electrode leads assembly 16 is depicted attached via disposable skin electrodes 18 to a patient's chest. The ECG cable and patient electrodes depicted in FIG. 2 are required for programmer functions requiring detection of cardiac and pacemaker signals as mentioned hereinbefore. However, the system may be used without the assembly 16 to program data into the implanted pacemaker or interrogate the pacemaker to telemetry-out program values and data. In current implanted pacemakers, confirmation of a programming transmission occurs by automatic transfer of data via telemetry-out. Use of the ECG cable and electrode leads assembly 16 is not required for programming confirmation with these devices. However, if telemetry is not received because of strong electrical interference, the programmer will attempt to confirm the transmission by surface detection of the PCI (program confirmation indicator) issued by the pulse generator. This method of program confirmation can occur only if the programmer is connected to electrodes on the patient. As described hereinafter, improvements of the present invention are directed toward the reduction or elimination of failures in telemetry-out because of strong electrical interference.
Furthermore, advanced programming and telemetry systems envisage the transmission of the patient s electrogram directly from the pacing electrodes in contact with the heart to the programmer via the RF telemetry link in order to provide near-field and/or far field electrograms directly from the myocardium. In such systems, the effects of electrical interference and other noise on the telemetry transmission is of concern. Medtronic U.S. Pat. No. 4,556,063 (incorporated herein by reference) describes such a system for telemetering out both digital and analog data.
In order to initiate and complete all programming and telemetry functions, the programming head 14 must be properly positioned over the pulse generator so that the POSITION HEAD indicator lights go out FIG. 3 depicts the positioning of the programming head 14 against the patient's skin overlying the implanted pulse generator. The optimum positioning of thelprogramming head over the implanted pulse generator 20 is depicted in FIG. 4. In use, after the programming system is coupled together as depicted in FIGS. 1 and 2 and the pulsegenerator model is keyed into the keyboard of the programmer 10, the POSITION HEAD indicator LED 22 lights up and remains lit until a telemetry link is established between the telemetry head 14 and the implanted device 20. The indicator LED 22 goes out to indicate when the programming head is properly positioned over the implanted house generator 20. For all programming and telemetry functions, the programming head 14 must be positioned so that the POSITION HEAD indicators go out.
In the system depicted in FIGS. 1 through 4, while the programming head is held in position over the pulse generator 20, the POSITION HEAD indicator is periodically flashed each time the programmer 10 receives an invalid interval and adjusts the telemetry signal sensitivity for best reception. If the telemetry link is lost due to programming head reposition or interference, the indicator LED 22 comes on steadily. While the POSITION HEAD indicator LED 22 is on, the programming head 14 may emit a radio frequency "search" signal used to establish a telemetry link with a pulse generator 20. It is desirable to avoid prolonged application of the RF search signal since the search signal can be sensed by the pulse generator and could occasionally cause unintentional inhibition or triggering of the pulse generator 20.
The pulse generator 20 contains a reed switch responsive to the externally applied magnetic field of the programmer head 14 and an RF signal transceiver circuit for receiving, amplifying and processing telemetered-in programming or interrogation commands. Actuation of the pulse generator reed switch and signal transmission and reception is affected by the programming head position and its distance from the pulse generator 20.
The presence of electrical interference or noise strong enough to interrupt reception of telemetry from the implanted pulse generator can affect operation of the POSITION HEAD indicator lights and telemetry functions, including programming confirmation. In such cases, the POSITION HEAD indicator LED 22 may not go out, or a "TOO MUCH INTERFERENCE" or "NO TELEMETRY" message may appear in the LCD following use of a programming or telemetry function.
Hospital operating rooms, catherization laboratories and even physicians' offices are often noisy electrical environments, and such noise has been found on occasion to interfere with the proper programming or interrogation of an implanted pacemaker. Such programming and interrogation of the implanted pacemaker's function is commonly undertaken as part of the surgical implant of the implanted pulse generator. Other equipment in the operating room or in adjoining rooms or floors of the facility may generate severe electrical noise. To ensure the safety of the patient, prior systems, as explained above, have been designed conservatively to avoid misprogramming once acquisition has been obtained by closure of the reed switch of the implanted pulse generator.
To minimize the effects of strong electrical interference, the Model 9713D programming head is provided with a manually controlled, five position, telemetry receiver gain control. The instructions for use with the Model 9713D programming head indicate that in the initial operation of the programmer, the gain control should be set to its highest gain to provide the most sensitive setting. However, if the POSITION HEAD indicator lights did not go out, and if several repositions of the programming head are unsuccessful, then the user is instructed in the physician's manual to reduce the receiver sensitivity by turning the gain control to a lower setting and again repositioning the programming head for each gain setting until a setting is found that provides proper operation. Similarly, if during the use of the telemetry function or confirmation of a programming transmission, the message "TOO MUCH INTERFERENCE" or "NO TELEMETRY" appears on the programmer display, then again sensitivity is to be reduced until a setting and positioning of the programming head is found that provided proper operation.
In response to the above-described disadvantages of the prior art system, attempts have been made to provide automatic gain control circuitry (see for example, U.S. Pat. No. 4,562,840) and software subroutines e.g., those within the Medtronic MemoryMod cartridges, described hereinafter as the "old algorithm" and specifically disclosed in Medtronic U.S. Pat. No. 4,531, 523, incorporated herein by reference. As mentioned hereinbefore, the POSITION HEAD indicator light LED 22 on the programming head 14 indicates proper positioning of the programming head 14 over the implanted pulse generator 20 by going out. This is accomplished by resident software within the MemoryMod cartridges that monitors the intervals of time between RF pulses received by the programmer.
As described in Medtronic U.S. Pat. No. 4,542,532, incorporated herein by reference, the RF telemetry pulse signal is centered at 175 kHz with a band width of 25 kHz. The RF signals are pulse interval modulated with a "1" or "0" interval value. These intervals are monitored on a continuous basis by one of the processors in the programmer. If any valid interval is detected as within the valid range (732.+-.15% micro seconds for a binary "0" and 1098.+-.15% micro seconds for a binary "1"), a flag is set indicating this state. If any interval is received outside of the valid ranges, the flag is reset. Once every 250 ms., another processor within the programmer examines the contents of this flag. If the flag is set indicating the valid interval collected, the POSITION HEAD LED 22 is turned off. If the flag indicated an invalid collected interval, the LED 22 would be turned on. This POSITION HEAD indicator LED 22 is used by physicians to validate the condition of the link prior to requesting information from the IPG. The information uplinked by telemetry from the implanted pulse generator 20 lasts 300 ms., while the test to determine the validity of the link checks lasts 1 ms. and indicates a good or bad telemetry link, depending on the state of the single collected interval prior to the most recent 250 ms. check.
Moreover, the old algorithm set the automatic gain control at maximum to start telemetry-in and operated in an essentially uni-directional fashion to decrease the maximum gain under certain conditions to be described hereinafter. The old algorithm disadvantageously tended to leave the system operating at high gain and thus susceptible to the detection of noise in a noisy environment.
Even with these improvements incorporated in the prior art systems described above noise interference is still encountered. The physician is still instructed in the physician's manual to either identify the source of interference and eliminate it or move the patient and programmer to another location to avoid its effects.
As mentioned hereinbefore, the prior system provided the user with the POSITION HEAD LED 22 and two visual commands, or prompts, in response to weak or noisy signal reception from the implanted pulse generator 20. It would be desirable to provide the physician with a more effective automatic gain control algorithm and with a more precise indication of signal strength in relation to the then prevailing AGC setting, in order to ascertain the cause of difficulties in programming or telemetry.